1. Technical Field of the Invention
The present invention relates to ethernets, and more particularly, to gigabit ethernet line drivers.
2. Description of the Related Art
Ethernet standards 10BASE-T and 100BASE-TX have become dominant in Local Area Networks (LAN) applications due to the low cost and wide use of transmission media based on unshielded twisted pair cable (UTP). The development of these standards has resulted in an emerging 1000BASE-T standard, which provides a significantly higher data transmission rate. The higher transmission rate has been achieved not only by doubling the number of cables of the transmission lines, but by also implementing a full duplex transmission mode with multi-level signaling.
Compared to the above-mentioned standards these features have dramatically changed the parameter requirements of the receiver and the transmitter. The multilevel signaling requires substantially more linear performance and lower noise generation from both the transmitter and the receiver. Full duplex operation requires an accurate hybrid to separate the received signal from the transmitted one at the input of the receiver. The hybrid output still contains some leftovers from the transmitted signal as well as additional signals reflected from different parts of the cable and cross talk signals coming from other cables. All these signals known as echoes are usually removed from the received signal by a DSP echo canceller incorporated in the receive path of the transceiver. The effective operation of the echo canceller depends on the level of non-linear distortions and noise generated in the analog portion of the transmit and receive path of the transceiver. Hence not only the hybrid should be added to the analog blocks used in the earlier standards, but the parameters of the existing blocks need to be substantially improved to be suitable for 1000BASE-T standard.
One possible solution is presented in U.S. Pat. No. 6,259,745. The driver is made of a plurality of switchable current Digital-to-Analog converter (DAC) cells. In order to reduce the high frequency noise generated by the combination of the current switches and the parasitic inductance and capacitance of the chip package and the transformer, the driver output should be filtered. Because the driver produces a significant current, and the input impedance of the transformer connected to the transmission line together with the matching resistance is just a few tens of Ohms, the capacitance of the driver output filter must be very large and is not well suited for location in the transceiver chip. The hybrid is created as a smaller replica driver, which produces a current proportional to the current of the driver. The hybrid current is applied to a pair of resistors connected to the driver outputs. This partially cancels the transmitted signal voltage at the receiver input. In this architecture, the hybrid is controlled by the same digital input and clock used in the driver portion of the transceiver. To get a good compensation of the transmitted signal the hybrid output filter should be well matched with the output filter of the driver.
A different solution is proposed by Roo, et al. “A CMOS Transceiver Analog Front-End fro Gigabit Ethernet Over CAT-5 Cables,” 2001 IEEE International Solid-State Circuit Conference, Session 19, Digest of Technical Papers, page 310 (2001). In this driver, a DAC is coupled to the transformer through a current mirror with a substantial current gain. This enables the use of smaller filter capacitors on the DAC output. The active hybrid transistors have their gates connected to the same current mirror and better match the hybrid frequency response with the one of the driver. However, because of the nonlinear behavior of the MOS transistor transconductance, the charge accumulated at the filter capacitors together with the gate capacitors of the transistors introduces substantial distortions into the dynamic behavior of the transmitted signal. The trajectories of the driver output are not quite proportional for the signals of different amplitude. This can be considered as non-linear distortions. Although the gates of the hybrid transistors are connected to the gates of the driver transistors, the distortions of the transmitted signal leak into the receive path because of an imperfect match of the hybrid and driver. It creates a problem for a DSP echo canceller to remove this part of the echo. Because the canceller is usually a linear device, the distortions lift up the noise level and reduce the signal to noise ratio.
To reduce the non-linear distortions in the driver, the driver output transistor transconductance linearity must be improved. It can be done by increasing the operating current and introducing degenerating resistors to the MOS transistors source nodes. This leads to substantial loss of the power efficiency of the driver. Thus, there is a need for a power efficient driver with low-level, non-linear distortions for 1000BASE-T application.